In more than ninety years of exploration only three skeletal relics of man have been found in the ancient 'river-drifts'; these are the 'Trinil,' the 'Heidelberg,' and the 'Piltdown'; in each instance the human remains were buried accidentally with those of extinct animals, after drifting for some distance in the river or stream beds. It is only in late Acheulean times that human burial rites or interments begin and that skeletal remains are found. Owing to the less perishable nature of flint, relics of the quarries and stations are infinitely more common; they are found both in the river sands and gravels, in the 'river terraces,' and in the 'loess' stations of the plateaus and uplands. Thus prehistoric chronology is based on observations of the geologist, who in turn is greatly aided by the archæologist, because the evolution stages of each type of implement are practically the same all over western Europe, with the exception of unimportant local inventions and variations. In brief, the large divisions of time are determined by the amount of work done by geologic agencies; the comparative age of the various camp sites is determined by their geologic succession, by the mammals and plants which occur in them, and finally by the cultural type of any industrial remains that may be found. TIMES OF THE 'HIGH' AND 'LOW' RIVER 'TERRACES' The so-called 'terrace' chronology is to be used by the prehistorian with caution, for it is obvious that the 'terraces' in the different river-valleys of western Europe were not all formed at the same time; thus the testimony of the 'terraces' is always to be checked off by other evidence. [Illustration: FIG. 6. Terraces on either side of the valley of the River Inn, Scharding, Austria, formed by sand and gravel deposits partly covered with loess. After Brückner. _Ib._ Very broad river deposits of First Glaciation, on the first erosion level, covered with the 'Upper Loess' of the Second Interglacial Stage. _IIb._ Somewhat narrower river deposits of Second Glaciation on the second erosion level. _IIIb._ Still narrower river terraces of the Third Glaciation on the third erosion level, covered with the 'Lower Loess' of the Third Interglacial Stage. _IVb._ Fourth or lowest terrace of the Fourth Glaciation on the fourth erosion level. _Va._ Erosion terraces, Achen. _VIa._ Post-Bühl erosion. _Loess′_, 'Upper Loess' of Second Interglacial. _Loess″_, 'Lower Loess' of Third Interglacial.]